In the ninja world, many shinobi use blood to assist in performing ninjutsu, but very few techniques absolutely require blood as a medium.
As a part of the body, blood is an excellent chakra conductor. In general, materials used for seals, enchantments, and barrier formations often contain blood from animals—or even humans—to enhance chakra flow and stability. When Mizuki performs Summoning Jutsu (Kuchiyose no Jutsu), he often uses his own blood to strengthen the link, especially because the summoned creature—his jellyfish—is of low intelligence and otherwise hard to control.
Another category is bodily-overdraft techniques, which forcefully draw on the user's life force, often squeezing blood or severely stressing the body's physical structure.
Beyond those examples, there's a darker class of jutsu centered around blood sacrifice. The most infamous example is the ritualistic technique used by Hidan of the Akatsuki. As a follower of Jashin, Hidan performs a gruesome sacrificial ritual using blood—his own or that of his victims—to create a deadly link between him and his target. Once the link is established, any damage he inflicts upon himself is transferred to the opponent, making it a rare and terrifying form of blood-based jutsu.
As for the principle behind Summoning Jutsu, Mizuki isn't sure if it parallels sacrificial techniques. However, he draws a comparison to the Aburame clan's insect-based jutsu. Their techniques involve forming a symbiotic contract where insects inhabit their bodies in exchange for chakra. This kind of mutual exchange bears some resemblance to the summoning contract, which might imply that summoning involves a form of spiritual or biological trade.
Thus, unless the summoned creatures are exceptionally weak—like Mizuki's jellyfish—it's unlikely that they would fight alongside a human for nothing. High-tier summoned creatures are intelligent, sentient beings. The most well-known examples—Mount Myōboku's toads, Ryūchi Cave's snakes, and Shikkotsu Forest's slugs—are near-mythical beings who exist at the pinnacle of power in the ninja world.
If the world were to fall under the sway of a threat like Kaguya Ōtsutsuki, using the Infinite Tsukuyomi to enslave all humanity, how would that matter to them? The toads would still eat insects, the snakes would still devour frogs—why would they care who the "Child of Prophecy" is? Why involve themselves in human wars at all?
If there's one technique where sacrifice is undisputed, it's the Dead Demon Consuming Seal (Shiki Fūjin). In exchange for summoning the Shinigami, the user must forfeit their soul to seal their enemy's. It is the ultimate price.
Summoning Jutsu, as the name implies, is a form of communication with another being. It might not operate under the same principle as the Shiki Fūjin, but there's a spiritual logic to it. What Mizuki still doesn't understand is—what do the summoned creatures gain from humans?
"Could it be," he wondered, "that they don't benefit directly from the human summoner, but instead gain something from being tethered to this world? If that's the case, it makes a certain kind of sense—but I can't confirm anything without examining a genuine summoning contract scroll."
With that thought, Mizuki picked up his research notes on summoning again and scanned them carefully, but found nothing new.
"It looks like I'll need to conduct some real-world testing. I can't make assumptions based on theory alone."
The next morning, Mizuki checked on his clone, "Zhuli," who was still recovering from injuries. His condition had stabilized and was no longer critical. That high-speed escape technique—an unstable, incomplete version of space-time teleportation—had been used as a desperate measure.
Before creating the clone, Mizuki's summoning technique had worked normally. But now, it failed. Upon inspection, the reason became obvious: Summoning Jutsu functions between two distinct individuals. Since the contract recognizes Mizuki and the clone as the same entity, the technique no longer activates correctly. At best, he could only establish a sensory link.
Had it not been for this limitation, Mizuki wouldn't have rushed to develop spatial ninjutsu. But creating true space-time techniques turned out to be far more difficult than expected. Only now did he fully appreciate how formidable the Flying Thunder God Technique (Hiraishin no Jutsu) truly was—Minato's ability to bend space using reverse summoning was nothing short of genius.
At Konoha Hospital, many injured Chūnin Exam participants were being treated. Mizuki, currently without an assigned mission, wasn't being bothered. As he walked through the halls, he passed rooms where covered bodies were wheeled out—casualties of the exam. Some of them, ironically, had been saved by Mizuki himself.
On the top floor, he reached the ICU. Kakashi was already waiting.
"What's with the secret summons, Kakashi?" Mizuki asked, noting the presence of ANBU stationed nearby.
"As you can see, Sasuke's in rough shape," Kakashi said. "But in one month, he has to compete in the final round of the Chūnin Exams. I need to give him some special training—partly for protection, partly to prepare him."
"So?" Mizuki raised an eyebrow. "Are you calling me here just to brag about your star student?"
Kakashi waved the jab off. "Listen. All three of my students advanced to the final round, and I can't train them all at once. I'm having Ebisu work with Naruto, since his fundamentals are still weak."
"Oh, so now you want me to train Sakura," Mizuki deduced.
"Exactly."
"You must be joking," Mizuki said flatly. "Ebisu's a tokubetsu jōnin who specializes in teaching. Why not assign her to him too? You're just being biased."
"Not exactly," Kakashi replied. "During the prelims, Sakura suspected the test was orchestrated by the village. She asked me who the examiner was."
"So you sold me out?"
"She asked directly. And she requested you specifically."
"Huh. That's unexpected," Mizuki mused. "Her opponent's that Oto-nin with sound-based attacks, right? The difference in power is pretty big. Is she planning something?"
"I couldn't say. I don't always understand what girls are thinking. But I really don't know anyone else suited to teach her."
"So I'm the fallback option?"
"Hardly. With your expertise in sealing, enchantments, and genjutsu, you're uniquely qualified. Sakura might not be ready for high-level techniques yet, but there's little difference between what you and I can offer her in the short term."
"Sealing and genjutsu, maybe. But enchantments?" Mizuki scoffed. "She'd be better off conserving stamina. Sound-based attacks are too unpredictable."
"Well, I'm counting on you. Teach her whatever you think is useful."
Mizuki sighed. "Since you've put it that way, fine—but don't expect me to babysit her for free. If I help, you owe me one."